1. Field of Invention
The present invention relates to means for improving the lamp current crest factor in electronic ballasts for gas discharge lamps, particularly by way of injecting a properly phased third harmonic component into an otherwise sinusoidal lamp current.
2. Description of Prior Art
A current's crest factor is defined as the ratio between its peak and RMS magnitudes.
An important characteristic of ballasts for gas discharge lamps relates to the crest factor of the current provided to the lamp. Improved lamp current crest factor means longer lamp life and improved efficiency. Most gas discharge lamps require to be powered with a current of crest factor no higher than 1.7.
In an inverter-type electronic ballast where the inverter is powered from a substantially ripple-free DC voltage, the lamp current is often nearly sinusoidal; which implies a crest factor of about 1.4--generally regarded as a very good crest factor.
However, in an inverter-type electronic ballast where the inverter is powered from a DC voltage having substantial ripple, the lamp current crest factor may be increased substantially beyond the point of 1.4--often to the point of substantially exceeding the highest crest factor permitted according to the specifications for most gas discharge lamps.
In fact, several currently available electronic ballasts exhibit lamp current crest factors of about 2.2.